The present invention relates to the compression of a refrigerant gas in a rotary compressor. Still more particularly, the present invention relates to apparatus for modulating the capacity of a rotary twin screw compressor.
Compressors are used in refrigeration systems to raise the pressure of a refrigerant gas from a suction to a discharge pressure which permits the ultimate use of the refrigerant to cool a desired medium. Many types of compressors, including rotary screw compressors, are commonly used in such systems. Rotary screw compressors employ intermeshed complementary male and female screw rotors which are each mounted for rotation in a working chamber within the compressor.
The male rotor has relatively thick and blunt lobes with convex flank surfaces. The female rotor has relatively narrow lobes with concave flank surfaces. The working chamber is a volume which is in the shape of a pair of parallel intersecting flat-ended cylinders and is closely toleranced to the exterior dimensions and shape of the intermeshed male and female rotors.
A screw compressor has low and high pressure ends which define suction and discharge ports respectively that open into the compressor's working chamber. Refrigerant gas at suction pressure enters the suction port from a suction area at the low pressure end of the compressor and is delivered to a chevron shaped compression pocket formed between the intermeshed rotating male and female rotors and the wall of the working chamber. Such compression pockets are initially open to the suction port and closed to the discharge port.
As the rotors rotate, the compression pocket is closed off from the suction port and compression of the gas begins as the pocket's volume begins to decrease as it is both circumferentially and axially displaced to the high pressure end of the compressor. Eventually, the compression pocket is displaced into communication with the discharge port through which the compressed gas is discharged from the working chamber.
Screw compressors often employ slide valve arrangements by which the capacity of the compressor is capable of being controlled over a continuous operating range. One such arrangement is the subject of U.S. Pat. No. 4,662,190 which is assigned to the assignee of the present invention. The valve portion of a slide valve assembly is built into and forms an integral part of the rotor housing. Additionally, certain surfaces of the valve portion of the assembly cooperate with the compressor's rotor housing to define the working chamber within the compressor.
A slide valve is axially moveable to expose a portion of the working chamber of the compressor and the rotors therein, which are downstream of the suction port and which are not exposed to suction pressure when the compressor operates at full capacity (with the slide valve closed), to a location within the compressor, other than the suction port, which is at suction pressure. As the slide valve is opened to greater and greater degrees, a larger portion of the working chamber and the screw rotors disposed therein are exposed to suction pressure. Such exposure to an area at suction pressure prevents the exposed portion of the working chamber and rotors, which would otherwise cooperate in defining a closed compression pocket, from engaging in the compression process. In effect, capacity reduction is obtained, through the use of a slide valve, by reducing the effective length of the rotors.
When the slide valve is closed, the compressor is fully loaded and operates at full capacity. When the slide valve is fully open, that is, when the portion of the rotors exposed to suction pressure other than through the suction port is at its greatest, the compressor runs unloaded to the maximum extent possible. The precise positioning of the slide valve between the extremes of the full load and unload positions is relatively easily controlled. Therefore, the capacity of the compressor and the system in which it is employed is capable of being modulated efficiently over a large and continuous operating range.
Still other arrangements for controlling the capacity of screw compressors are lift valve arrangements of the type described in U.S. Pat. Nos. 2,358,815; 3,108,740; 4,453,900; 4,498,849; 4,737,082 and 4,946,362. These patents suggest the use of various kinds of lift unloaders which, when opened, place what would normally be a closed compression pocket in communication with an area of the compressor which is at suction pressure. By doing so, that compression pocket volume is rendered incapable of being used in the compression process.
Such mechanisms are commonly referred to as step unloaders since the opening or lifting of each such unloader results in a reduction of compressor capacity in a discontinuous, stepwise fashion and by a discrete, predetermined and relatively large percentage of the compressor's capacity. Such arrangements do not permit the unloading of a compressor over a continuous range of capacities and therefore, while somewhat less complicated and expensive to employ than slide valves, do not provide the flexibility or energy efficiency of slide valve arrangements.
Next, screw compressor piston unloading arrangements of the type illustrated in U.S. Pat. Nos. 4,042,310; 4,544,333 and 4,565,508 are known and are characterized by the disposition of an unloading piston in a cylindrical bore within the compressor housing which is remote from the working chamber. The bore in such piston unloading systems is in communication with the working chamber through a series of axially spaced ports and is likewise in communication with an area of the compressor which is at suction pressure. When the unloading piston is positioned within the bore so as to completely interrupt communication of the bore with the compressor's working chamber through the ports, the compressor operates fully loaded since the axially spaced ports are closed and the working chamber is prevented from communicating with any portion of the compressor which is at suction pressure other than through the suction port.
The unloading piston is capable of being moved axially within the bore to fully or partially uncover the axially spaced ports communicating between the bore and working chamber thereby providing for the unloading of the compressor by the selective opening of the ports. This type of piston unloading arrangement, while providing for more continuous and precise slide valve-like capacity control than a step unloader arrangement, can be more expensive and difficult to implement than step unloading arrangements.
Further, the re-expansion volumes associated with the unloading ports of such piston unloading arrangements, particularly if compressor unloading over a large capacity range is desired, becomes excessive. In that regard, it is noted that the effect and performance penalty associated with the existence of such re-expansion volumes is far more pronounced at the discharge end of the compressor where the pressure in a compression pocket becomes significantly elevated. It should also be noted that unlike piston unloading arrangements, the use of a slide valve or step unloaders does not result in the creation of re-expansion volumes since certain of the faces of their moving members form part of the working chamber wall and conform precisely to the adjacent outer contour of the rotor set.
While slide valve arrangements are preferred, particularly for their capability to match actual load and provide for continuous as opposed to step unloading, they do bring with them certain inherent leakage paths and losses because of the manner in which surfaces of the valve function to define a portion of the wall of the compressor's working chamber. In that regard, such surfaces interact with the lobe tips of the screw rotors to define the closed compression pockets previously referred to. The clearance between the tips of the rotor lobes and such slide valve surfaces is a leakage path which is inherent in any slide valve arrangement.
In larger capacity, more expensive screw compressors, which "compete" for use with relatively expensive centrifugal compressors, leakage past the rotor/slide valve interface is of proportionately lesser significance. Further, the expense associated with a slide valve arrangement in larger systems is more than made up for by the versatility and energy efficiency offered by slide valve unloading systems which are capable of precisely matching compressor capacity to system load.
In smaller screw compressors and systems, however, which "compete" for use with less expensive scroll and reciprocating compressors, the inherent leakage associated with slide valves is proportionately and unacceptably higher as is the cost associated with their use so that their use in small capacity compressors is uncommon. The use of step unloaders alone in smaller screw compressors, while quite common and competitive with unloading arrangements for scroll and reciprocating compressors, brings with it the penalty of a relatively inflexible and unsatisfactory unloading capability given today's demand for efficiency in energy consuming products.
Further, because certain screw rotor profiles are such that the male rotor lobes are quite "thick", with relatively little volume between them, the use of a lift piston step unloader at the discharge end face of such male rotors is not practically feasible. This is because the size of the port through which unloading must occur is insufficient, given the thickness of the lobes and the rotational speed of such rotors, to permit all of the gas to escape through the port while the port remains open. It is noted that lift piston step unloaders disposed at other than the end face of a rotor can effectively be used although unloaders such as those are disadvantageous from the standpoint that they are more costly to manufacture and tolerance critical to the extent that the end face of the unloader is a curved surface rather than a flat face or to the extent that the use of a flat face unloader results in the creation of re-expansion volume.
Likewise, the use of an axial piston unloader arrangement over the preferred full range of unloading, particularly in a smaller capacity screw compressors, is not practically feasible for high efficiency compressors. This is, once again, because the nature and number of the ports communicating between the remote bore in which the piston is disposed and the working chamber, when such an arrangement is exclusively used over a large unloading range in a small compressor, is such that the compression losses associated with the ports, which in effect are re-expansion volumes (i.e. volumes which are not used in the compression process) can become unacceptably large particularly when located in a high pressure region of the working chamber where the re-expansion effect is significantly more pronounced.
The need therefore exists for an unloading arrangement for screw compressors which is amenable for use, even with smaller capacity screw compressors, when cost, leakage, efficiency, flexibility and manufacturability factors are taken into account and particularly, when compared to competitive non-screw compressor based arrangements which are relatively inflexible and energy wasteful from the unloading standpoint.